I opened the heavy door and went inside.
新しい扉が開かれた。
A new door has been opened (a new opportunity has arisen).
Specifically the title page of a book or publication. Less common than the 'door' sense.
本の扉には著者の名前が書いてある。
The author's name is written on the title page of the book.
He opened the car door and got in.
It's cold, so please close the door.
The door opened automatically.
That discovery opened the door to a new era.
I locked the door before going out.
front door (cabinet, bus, etc.) · half-title; bastard title
external door; outer door